Principles

In designing and building for low heat loss, both good insulation and control of air infiltration are needed. Good attention to detailing is necessary during installation for insulation to work effectively and to ensure unwanted air infiltration is eliminated as far as practicable. This translates into "thermal continuity of the insulation" and "air tightness of the building".

How to achieve thermal continuity - and why

For thermal insulation to be effective, it needs to be continuous. This means no gaps between the insulation sheets or batts. It also means no way for cold air to circulate freely on the warm side of the insulation.

Insulation boards with stepped rather than flat butt joints give better continuity.
Cut cavity insulation to suit. Butt the sheets tightly to each other, as well as tight up against cavity closers and loose fill insulation.
Install roof insulation over the top course of blocks at the eaves, prior to felting at the roof having brought the wall insulation up to the top of the wall and bring the wall insulation right up to the top.

Good practice

Use of an air tight membrane as the air barrier can further increase air tightness beyond the performance of the barrier options given in the diagrams. To limit condensation, it may be helpful to restrict the contact of non-breathable insulation on timber studs, joists or other sections to less than 50% of the section perimeter. With internal dry lining, a vapour barrier to prevent interstitial condensation on the structure is particularly important. To reduce cracking and help air tightness, it may be helpful to tape mesh onto wall/ceiling junctions in advance of plastering. Other good practice notes are provided on diagrams for specific junctions.

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Figure 1 - Gaps in insulation at critical junctions result in cold bridges, visible under thermal imaging

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Figure 2 house on which thermal image is based

How to achieve air tightness - and why

Air tightness means cutting out unwanted draughts. Draughts can be so slight as to be imperceptible, but even slight draughts increase heat loss, sometimes dramatically.

The way to good air tightness is a continuous air-resistant layer all around the inside of the building. This includes under and around the ground floor, across the external walls and under the roof, to seal the inside from the outside. With masonry walls - whether concrete block or concrete - this is most easily done by using a wet plaster finish. It can also be done by using dry-lining boards and by taking extra care to seal around all gaps, all perimeters, and at windows and external doors.

With timber frame or steel frame walls, it's most easily done by using plasterboard board with perimeters and joints all thoroughly sealed.

Points to watch:-

Plaster between the joists at suspended timber floors
Make sure there's no gap along the skirtings at floor level
Where pipes or wires pass through the outside wall or the roof, seal around them to draught-proof the opening
Tape around window and external door frames to stop the draught at the edges

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Figure 3 - Sealing the junction between the joist and the external wall

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Figure 4 - Sealing where pipes enter the roofspace

Ground floors

Thermal continuity

Concrete ground-bearing floors: The insulation under the floor slab must be continuous. This is easily installed and easily checked. This done, the only place to worry about is at the junction of the floor slab and the external wall.

Concrete suspended floors: The insulation is usually on top of the concrete and under a screed or a floor finish. As with ground-bearing slabs, this is easily installed and checked. Concrete suspended floors are often used on sloping sites. For this reason, there can be a significant amount of exposed external wall below the slab. The detail of the junction at the external wall then becomes key.

Timber floors are of their nature, suspended with a ventilated air space underneath. The potential for gaps in insulation between floor joists and a quilt is large. With well-built floors and for good thermal performance, insulation needs to be continuous between, under or over the joists, or a combination of these. Pay particular attention to potential gaps along joist edges.

Air tightness

Concrete ground-bearing floors: When properly installed, both the floor slab and the radon barrier give excellent air tightness. The junction at the external wall is key. Make sure the radon barrier is carried up the side of the external wall and across to the inner leaf. Then appropriate air tightness sealant or tape behind the skirting to seal between the radon barrier and the wall plaster or plasterboard.

With timber frame external walls, it's essential to maintain air barrier continuity between the wall construction and the ground floor slab. See the details sheets and below.

Concrete suspended floors: A well-cast concrete floor slab is airtight. As with ground-bearing slabs, the junction at the external wall is key. A properly built masonry inner leaf built off the slab will be air tight also. If the wall is timber frame or is not built off the slab, there's potential for a gap. As with a ground-bearing slab, use sealant or tape behind the skirting to seal to the wall plaster or plasterboard.

Timber floors: Boarding is prone to shrinkage. This can render the floor leaky. Fixing plywood sheeting across the boards and taping gaps can create an air tightness barrier. A continuous sealed floor finish can also provide an air tightness barrier.

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Figure 5 - Bonding an air-tight membrane to the concrete floor slab

Masonry walls

The majority of dwellings in Ireland are built with masonry external walls. These walls may be cavity walls, with inner and outer leaves of blockwork (medium density or lightweight), brickwork or concrete, with a cavity which is usually insulated, and frequently with additional insulation on the inner face. Alternatively, they may be built of single-skin masonry, frequently of hollow blockwork or of precast or insitu concrete, with insulation applied internally, sometimes externally, or sometimes both.

For all these, a well-built blockwork inner leaf with a coat of wet-finish plaster, will, if properly applied and with proper detailing, deliver air tightness. Properly applied dry-lining boards applied to the inside face can also provide air tightness. For enhanced performance, use a purpose-made airtight membrane. However, the issues surrounding continuity of thermal insulation and continuity of air tightness at openings, roofs and suspended floors vary widely between these wall types. The details in Section 2 show these issues in detail. The following identifies key principles.

Thermal continuity with single-skin masonry external walls

Internally applied insulation (insulated dry-lining) needs to be done carefully to achieve thermal continuity. Pay particular attention to gaps at the tops and bottoms of boards, at floors and ceilings and around opes.

Significant advantages of externally applied insulation are its ease of application and also of checking its continuity. Tightly-butted or lapped sheets deliver thermal continuity with little difficulty.

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Figure 6 - Internal dry-lining on blockwork with thermally efficient fixings

Thermal continuity with cavity masonry external walls

Well-built cavity walls have clean cavities, with cavity insulation held firmly against the inner leaf. Insulation sheets with lapped or tongue-and-groove edges, or which are of fibrous type, can be butted tightly against each other and give reasonable thermal continuity.

Clear all debris including mortar snots from cavity as work progresses to prevent thermal bridging between the inner and outer leaf.
Cavity insulation should be cut to suit. Tightly butt sheets to each
other and to surrounding cavity closers.
Fix insulation tight to the outer face of the inner block work leaf, to prevent air circulation between the block and insulation reducing the performance of the insulation layer.

Air tightness with cavity masonry external walls

The nature of cavity construction makes it difficult to achieve good air tightness by sealing externally. The simplest way to achieve good air tightness is to plaster the inner leaf. Dry lining with proper sealing of all perimeters and joints will also achieve good air tightness. The key areas to watch are junctions at opes, at floors, and at service penetrations .

Timber and steel frame

Thermal continuity with timber and steel frame external walls

External walls of timber frame usually have insulation fitted between the load bearing studwork in an inner leaf, possibly with additional insulation applied to the inner face of the studs.

Well-built timber frame walls have sole plates tight to the masonry underneath, with insulation fitting snugly into the space between each pair of studs and the ply sheathing outside.

Ensure the insulation is cut to fit snugly into the space betwee each pair of studs and the ply sheathing outside.
Fill the entire stud depth with insulation
Fit a second layer of insulation inside the studs to lap over the studs

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Figure 7 - Tightly-fitting insulation in lightweight frame

Air tightness with timber and steel frame external walls

The internal plasterboard layer provides air tightness and is simply executed. Sealing the junctions of the plasterboard with the surroundingconstruction is key. This includes at intermediate floors, roof and
ground floor, external wall opes and service penetrations. For best practice, use an air-tight membrane as illustrated.

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Figure 8 - Joints sealed in air tight membrane

Intermediate floors

Thermal continuity with timber intermediate floors

If the thermal insulation is in the cavity or is external type, thermal continuity at the junction of the intermediate floor and the outside wall happens almost of its own accord. So long as the cavity insulation is continuous across the joist hangers, continuity is achieved.

If the insulation is on the inner face of the external wall, thermal continuity requires greater attention to detail. There is a potential cold bridge all along the zone of the suspended floor.

If the insulation is on the inner face of the external wall, make sure that insulation is carried up (or down) between the floor joists and tight to the timbers on all sides. Continue the insulation up the wall from below and make sure it's continuous at ceiling level.

Thermal continuity with concrete intermediate floors

As with timber floors, if the thermal insulation is in the cavity or is the external type, thermal continuity at the junction of the intermediate floor and the outside wall is achieved readily.

If the insulation is on the inner face of the external wall, thermal continuity is not possible.

Air tightness at intermediate floors is a matter of closing the gaps above and below where floor spans onto the external wall, and around any joists, beams or joist hangers.

In timber floors, where joists run parallel to the external wall, or when hangers are used for joists requiring support, air tightness is achieved by bedding the hangers in mortar, and by plastering the external wall the same as is done for the rest of the wall. With timber frame or with dry-lined masonry, carry the boards into the floor zone and tape around the joists or hangers, see below. Where timber joists span onto the external wall, carry the boards into the floor zone and tape around the joists or hangers, as per Figure 10.

With concrete intermediate floors, when the floor spans onto the wall, pay attention to any gap under the slab, especially with precast concrete slabs. If a blockwork wall is built off the floor slab above, this will give an excellent basis for air tightness once the blockwork is plastered right down to the slab.

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Figure 9 - Sealing at intermediate floors: First stage: point up around joists

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Figure 10 - Sealing at intermediate floors: Second stage: tape joists to wall

Separating wall junctions

The concern at separating walls is the structural continuity which is usual between the separating wall and the exterior wall. This can result in breaks both in thermal insulation and also in air tightness.

Thermal continuity

The issues which arise are similar to those with intermediate floors or with staircases. With a masonry structure, insulation in a cavity, or exterior insulation, both deliver thermal continuity. This is because the insulation runs uninterrupted either externally or in the cavity and outside the junction of the walls.

With an internally insulated masonry structure, the insulated dry lining needs to be returned for at least 1 metre along the separating wall.

Air tightness at separating wall junctions

Where a separating wall is of cavity construction, and where the cavity is joined to a cavity in the external wall, close off the cavity paths to cut down on air routes. This can often be done using the fire stopping required under Building Regulations Part B or under the flanking sound requirements of Part E.

Windows and external door opes

Thermal continuity at window and external door opes

Correct choice of the lintel or lintels to be used when forming an ope in an external wall is a key factor in ensuring thermal continuity. The selection of the method of closing the cavity at the jambs, and the detail of the cill or threshold, are equally important. The non-repeating cold bridges at these locations can account for a significant degree of heat loss in an otherwise well-insulated building.

For good thermal performance:

Use separate lintels and insulate between them.
Fill all gaps around and between lintels with tightly packed insulation. Overlap the frame and this insulation by at least 30 mm.
Secure any partial fill insulation firmly against the inner leaf.
Cut cavity insulation to suit. Sheets should be tightly butted to each other and surrounding cavity closers and loose fill insulation.

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Figure 11 - Internal insulation carried well up to the external wall ope

Air tightness at window and external door opes

Air leakage often occurs between window or door frames and the surrounding construction. Appropriate air tightness sealants are required between plaster finishes, window boards and frames. This also applies to internal door frames (particularly the architrave over th door head) where air leakage may enter the wall lining void and track to the external cavities. Approved air tightness sealants and tapes are available to assist the formation of air barrier continuity at such interfaces.

For air barrier continuity:

Apply flexible sealant or a certified tape to TGD D guidance at all interfaces between the internal air barrier and the window or door frame
If forming the air barrier to the walls with the blockwork inner leaf or a scratch coat on blocks, install an appropriate air tightness sealant between the cavity closer and blockwork wall.
Appropriate air tightness tapes can be used to seal between the wet plastered finish of the wall and the window frame.
Seal all penetrations through air barrier using an appropriate air tightness flexible sealant or tape.

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Figure 12 - Sealing under way at junction of timber frame wall and external wall ope

Trickle ventilators

Research has shown that if relative humidity levels exceed 70% for prolonged periods, there is a high probability that the condensation occurring on cold surfaces will lead to mould growth. A ventilation rate of between 0.5 and 1.5 air changes per hour (ach) for the whole dwelling will usually be sufficient to control condensation.

Permanently opened ventilators provide either fixed or controlled fresh air background ventilation to provide adequate air supply for occupants.These comprise of louvered ventilation openings located within the external walls or trickle ventilators located in the window frames.

Where wall mounted permanent ventilation openings (225 x 225mm vents) are used it is necessary to provide plastic ducting from the inlet wall mounted grille to the room permanently opened outlet vent to minimise thermal bridging and infiltration through each wall component as per 'Extract fan' specification.

Service penetrations

Holes and chases are formed for many different services by different specialist contractors. They may be in roof spaces (recessed light fittings, water pipes, soil vent pipes, rainwater pipes, ventilation ducts, television cables); in external walls (soil and waste pipes, electrical cables) and in ground floors (soil and waste pipes, incoming mains). Penetrations may also be required behind bath panels, shower trays, kitchen units and into service shafts.

A key element in maintaining thermal continuity and air tightness around service opes is to agree standard sealing procedures with subcontractors and make sure they have the right materials and tools.

Try to locate the following so as to minimise services and structure penetrations through the envelope

W.C. toilet overflows
Kitchen cooker hood extracts
Condensing boiler flues
Outside taps
Soil vent pipes
Waste pipes
Trickle vents in walls
Air intake vents
Canopies to entrances
Metal balconies
ESB connections and meters
Gas connections and meters
Security alarm systems
External security lighting
External security cameras and sensors

Thermal continuity and air tightness at service penetrations

For good thermal performance and air tightness:-

Core drill service penetrations to minimise damage to the insulation layer.
Make good any damage caused to the insulation layer by filling any gaps with loose fibrous insulation or approved expanding foam.
Drill holes to provide a snug fit and reduce oversize to a minimum.
All penetrations through the air barrier line must be effectively sealed following installation of the services. This can be achieved with the use of appropriate air tightness tape, air tightness
grommets or air tightness sealants.

When installing socket outlets or switch plates in an air barrier formed by a plasterboard lining, apply a continuous ribbon of dabbing sealant compound around the hole and metal electrical enclosure prior to installing the plasterboard, as this will ensure good seal between box and plaster board and the cable penetrations and the metal box, providing greatest structural integrity for this application. This will reduce air leakage through the sockets / switches into the void beyond.

Consider using proprietary gasketted socket boxes and membranes.

Construction of a services zone inside the air tightness barrier can also reduce the number of penetrations in the barrier.

Recessed light fittings may permit air leakage to breach the ceiling line into the voids or attics beyond. They should never be allowed to penetrate the primary air barrier unless the unitsare of an air sealed type or a further secondary air barrier formed beyond. This needs a special detail because of the fire risk.

Extract fans should be installed and sealed to prevent air leakage occurring through plasterboard finishes. A continuous ribbon of adhesive should be installed around the duct penetration at the air tightness barrier. Where possible the ducts should also be sealed to the blockwork inner leaf. Extract fans may also be fitted with external flaps to minimise air infiltration through the unit.

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Figure 13 - Sealing around connection to electrical socket outlet

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Figure 14 - Ply backing on lightweight frame, to hang services and minimise penetrations through insulation and air tightness barrier

In the roof

Discontinuity between wall and roof insulation at eaves / verge

Roof insulation should be installed to minimise the effects of thermal bridging at the eaves. Attention must be paid to the timing of installation of insulation at the eaves to ensure that it is effective as it is impractical to install when the roof has been completed The insulation should be laid over the top course of blocks and the wall insulation installed right up to the top of the wall.

Thermal continuity under the attic

For best practice, in cold roof spaces, use insulation over the ceiling joists, to eliminate the cold bridge otherwise caused by the joist.

Air tightness under the attic

Proprietary attic trap doors with low air permeability characteristics should be fitted in lieu of site manufactured doors. Where site manufactured doors are installed these should be complemented with draught stripping to minimise air leakage into the attic space above.

Dormers

The plasterboard cheek linings will form the air barrier. The linings should form a continuous air barrier and be sealed to the window frames. Proprietary products are available to assist the formation of air
barrier continuity at such interfaces.

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Figure 15 - Sealing around frame of access hatch to attic

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Figure 16 - Roof with ventilated counter battens and unventilated attic

Achieving Thermal Continuity and Air Tightness